How To – Fit a K&N Filter and Rejet a Carburetor

This article ‘How To Fit a K&N Filter and Rejet a Carburetor’ was published long back. We are bumping it up for a generic know-how for our new visitors…

This post is for all my fellow bike riders who wish to extract the last possible juice out of their ride. This post is regarding to the fitting of free flow air filters, its effects and how to rectify it using the method of re-jetting.

You need to consider re-jetting your bike whenever there’s been a major change to the air intakes (like installing K&N filters which allow more air to pass to the carburetor), or adding drag pipes. Both modifications will lean out the air-fuel mix. The stock jet can only allow a certain amount of fuel to pass through and is designed according to the specification of the vehicle. Installing larger jets increases fuel to the carburetor to restore the 14 parts air to 1 part fuel optimum mix. This mixture is called stoichiometric ratio. Whenever the amount of air intake is increased, the fuel part is also to be increased. In all other cases, the stock jets will take you through 95% of your riding requirements.

rejet a carburetor

Since the main jet effects all speed ranges after idle, you need to get the main jets right first, and then tune the mid-range and low end. All tuning should be done on a fully warmed up engine because that’s the way you usually ride; the spark plugs should have been previously checked and cleaned; the gas tank should be at least half full; a new or recently installed fuel filter if present; the air intake filters checked for holes, and cleaned; and all air intake and manifold connections tight and secure; the battery fully charged; and the carburetors synchronized, if it’s a multiple cylinder engine for optimum results.

We are now going to tune the carburetor without the help of a tachometer (engine speed). High speed or top end main jet sizes are determined by full open throttle performance. Low and mid-range tuning is determined mainly by how the bike feels to you as you roll the throttle. Mikuni CV carburetor is different from the Keihin CV. So is the difference with other carburetors. So tuning will be different in different vehicles.

Note: If you do sense a loss of power, or rough running on a stock vehicle, it may be due to some other reason. In this case re-jetting is not the solution to the current problem. Please take the bike to an authorized mechanic and rectify the problem.


High speed performance (80 KPH and up) is controlled by the main jets. To make sure you have the right main jets, take your bike out to an open highway where you can safely (and legally) open the throttle all the way in top gear. Ride the bike full open and let the bike get to top speed. Again, do this only if you are experienced and comfortable running at top speed and you are not in danger of injuring yourself or someone else.

A. If at top speed or before, the bike runs rough, heavy throttle feel or becomes hesitant and buck, your main jet is TOO BIG. The mix is too rich (more fuel than the optimum 1 part fuel to 14 parts air) and you are getting an uneven burn and poor performance. Install smaller main jets and take the bike for another test run. Another way to determine rich mixture is, when you cold start the vehicle and if the bike starts in the very first crank the mixture is rich (no need to use the choke in this instance). Low fuel efficiency and more smoke in the exhaust than usual are some of the characteristics of a rich mixture.

B. If before top speed the engine is running smooth, but has no power, seems to be running hot or knocking from the engine and you can’t push the bike to top speed, then your main jets are too SMALL. You are running too LEAN a mix (less fuel than the optimum 1 part fuel to 14 parts air) for maximum engine performance. Put in LARGER main jets and take the bike out for another test run. Another way to determine lean mixture is the difficulty you feel when cold starting. Even after repeated cranking, if the engine does not start, the mixture is lean. When you close the throttle on a downhill, if you hears popping or cracking sound from the exhaust, it is again due to the lean mixture.

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The carburetor needle controls mid-range performance. The needle tapers down to a point that fits into the main jet. The needle is lifted upward (along with the slide) by the carburetor diaphragm as the air flow increases through the carburetor. As the needle moves upward it is withdrawn from the main jet, allowing more fuel to mix with the increased air volume. All that is needed to modify mid-range performance is to place (or remove) one or more small, thin washers between the diaphragm and the head of the carburetor needle where it’s held in the diaphragm. Adding washers effectively raises the tapered needle further out of the main jet at all engine rpm’s and allows more fuel to mix with air to create a richer mix. Removing a washer effectively leans out the air-fuel mix.

rejet a carburetor

There’s usually no need to replace the stock needles since they have proven over time to provide good performance and good gas mileage. If you do use a third party needle for increased performance, then anticipate lower fuel efficiency.

Mid-range tuning is accomplished mainly by how the bike performance feels to you. If there’s a smooth increase in power as you roll the throttle, then you’re there. If there’s a slow response or there’s no power, then the mix may be too LEAN and you may need to add one or more washers to richen the mix. If the bike accelerates with power, but feels rough, you may be too RICH and need to remove one or more washers to lean out the mix. Now take the bike out for a test spin.


Now you can tune for low end performance. If you are getting a poor pickup or hesitation off the line, or a lot of backfiring on deceleration, then you need more fuel through the pilot jet. You increase fuel flow through the pilot jet by turning the pilot jet screw OUT a half turn. It’s best to start at 2 1/2 turns OUT on the pilot screws and then increase the turns OUT 1/2 turn at a time, and go for a test run. Do these until you get the performance you want and/or there are no back fires on deceleration. If you go more than 5 or 6 FULL turns out, you will probably have to install LARGER pilot jets. Larger pilot jets are usually needed if you have removed the stock air intakes, air box and installed K & N filters which dramatically INCREASE air intake volume.

I am not posting how to successfully repair the carburetor because of its complexity for a normal person. And friends, please make sure that a professional mechanic is always nearby if anything goes wrong.

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If you get inconsistent running, fluctuations in power, you are probably running rich. Lean running is more likely to cut power completely. If you have poor power at small throttle openings and a surge at wider throttle then you may be running lean. Quick ways to see if you are running rich or lean.


If you bike runs better with the choke on even after a few minutes of warm – up, your original mixture might be lean in the lower rev range. This test does strange things to the upper rev range, so don’t use it at highway speeds.


Temporarily remove the air liter cover (to INCREASE air to the mixture), and go for a test ride. A well tuned bike (where the air and fuel mix is right on) will run pretty badly (i.e. mixture too lean) when you do this. But if the mixture was originally too rich, the bike will run better. Then u can fix the air filter cover and lean the mixture.

If your bike’s performance has a hitch or hesitation at certain speeds or you can’t tell if it’s rich or lean, experiment by leaning out the mixture which is the easiest way to find out – if the mixture was originally too rich, the bike will run better immediately. If the mixture was originally too lean, your bike will run worse immediately

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(Use the tuning techniques listed above to adjust the low, mid- and high speed ranges).

Now you have may have also understood why manufacturers stick to a particular setting and do not go for extreme tuning. Extreme tuning will call the need for extreme keeping up of it. Friends, it’s all about experimenting with different settings. Also if you find a good setting, don’t forget to check the fuel economy too. And always ride safe! 🙂

– Abiram Menon